In many applications, the input voltage of DC/DC converters can vary over a very wide range. In various telecommunications applications, nominal input voltages of 24 V and 48 V may have actual ranges of about 18 to 75 V. Even for international systems employing AC input voltages where the standard is commonly an input voltage of 110 V and 220 V, wide DC/DC converter input voltage ranges exist.
To compensate for the wide range of input voltages, most power converters operate in smaller subranges, which are usually about 18 V-36 V (24 V nominal) and about 36 V-75 V (48 V nominal). As a result, a customer generally inventories two products for the same output specification thereby increasing the inventory costs and the development costs for manufacturers employing the power converters. Unfortunately, there is not a single product that covers the entire voltage range for many applications.
In low power applications, flyback converters may be employed to cope with wide input voltage ranges. The flyback converters, however, suffer from low efficiencies due, in part, to the high voltage/current stresses on components therein caused by wide duty cycle variations necessary to handle the varying input voltages. For high power applications, various authors suggest using dual bridge converters to address the wide operating voltage ranges. Proposed topologies are essentially full-bridge converters with two two-switch forward inputs. At low input voltage ranges, the inputs are connected in parallel, and at high input voltage ranges, the two inputs are switched to a series connection. Through this topology, the effective input voltage is nominally maintained in both high and low voltage ranges. The primary drawback to this configuration is the extremely complex power stage structure and switching mechanisms necessary to accommodate the two ranges. Due to these complexities, the topology is not suitable for many applications such as low to medium power installations.
Accordingly, what is needed in the art is switchable power converter topology that operates over a wide range of input voltages.